integral to floating point conversion

[Guillaume Boucher via Digitalmars-d]

On Sunday, 3 July 2016 at 09:08:14 UTC, Ola Fosheim Grøstad wrote:
> On Saturday, 2 July 2016 at 20:17:59 UTC, Andrei Alexandrescu 
> wrote:
>> So what's the fastest way to figure that an integral is 
>> convertible to a floating point value precisely (i.e. no other 
>> integral converts to the same floating point value)? Thanks! 
>> -- Andrei
>
> If it is within what the mantissa can represent then it is 
> easy. But you also have to consider the cases where the 
> mantissa is shifted.
>
> So the real answer is:
>
> n is an unsigned 64 bit integer
>
> mbits = representation bits for mantissa +1
>
> tz = trailing_zero_bits(n)
> lz = leading_zero_bits(n)
>
> assert(mbits >= (64 - tz - lz))

This is the correct answer for another definition of "precisely 
convertible", not the one Andrei gave.